Field of the Invention and Related Art Statement
The present invention relates to an optical recording/reproducing apparatus having means for protecting an actuator included therein by periodically supervising a drive current supplied to the actuator.
As the optical recording/reproducing apparatus, there have been available an optical disk recording/reproducing apparatus and an optical card recording/reproducing apparatus. A conventional structure of the optical card recording/reproducing apparatus of the above-described type will now be described.
As shown in FIG. 1, an optical card 1 has a data recording portion 2 in which, guide tracks 3 are disposed at predetermined intervals as shown in FIG. 2 so that data is recorded to tracks 4 each of which is formed between adjacent guide tracks 3.
The optical card recording/reproducing apparatus comprises: focus servo means for controlling a recording/reproducing beam to be focused on the recording medium; and track servo means for controlling the recording/reproducing beam to be positioned at the central portion of the track. Therefore, the optical card recording/reproducing apparatus must use a focus error signal denoting the deviation from the focused point and a track error signal denoting the positional deviation from the central portion of the track.
FIG. 3 illustrates an example of the optical system of an optical head 30 for use in the above-described optical card recording/reproducing apparatus. Light beams emitted from a laser diode 5 are made to be parallel beams by a collimator lens 6 before they are sectioned into three beams by a diffraction grating 7. The beams are condensed onto the surface of the optical card 1 by a condensing lens 8 so that data is recorded/reproduced. In this state, the structure is arranged in such a manner that the axis of the beam made incident upon the recording medium and that of the reflected beam are deviated from each other with respect to the central axis of the condensing lens (objective lens) 8. The reflected beams are reflected by a mirror 9 before they pass through a detection lens 10 so as to be applied to a photo-detector 11.
FIG. 4 illustrates the photo-detector 11 and the beams received by the same. Referring to FIG. 4, a photo-detector element which receives a central beam (main beam) 15a of the three beams is sectioned into two portions.
Beams 16a and 17a (sub-beams) are respectively received by photo-detector elements 13 and 14 so as to be used to detect the track error signal.
When the beams are focused, the beam 15a is positioned at the central portion of the bisectioned photo-detector 11, the beam being shifted, due to the deviation of the axes, in directions designated by arrows when the interval between the condensing lens 8 and the optical card 1 is changed. FIG. 5 illustrates the beams positioned on the optical card 1, where the beams 15a, 16a and 17a shown in FIG. 4 correspond to 15b, 16b and 17b.
Therefore, as shown in FIG. 6, the output from a subtracter 26, which calculates the difference between the output from the photo-detector element 11a and that from the photo-detector element 11b, is made to be the focus error signal.
As for the track error signal, the output from a subtracter 18 which calculates the difference between the photo-detector elements 13 and 14 as shown in FIG. 7 is made to be the track error signal because the quantity of reflection of each of the beams 16a and 17a on the photodetector elements 13 and 14 from the tracks 3 is changed depending upon the positions of the beams on the tracks.
The condensing lens 8 is, by a lens drive portion 12, driven in directions respectively designated by arrows F and T shown in FIG. 3.
FIG. 8 is a block diagram which illustrates a track servo system according to the conventional technology.
The outputs from the photo-detector elements 13 and 14 are subjected to the subtraction operation performed by the subtracter 18 so that the output from the subtracter 18 is made to be the track error signal, the track error signal acting to drive a track actuator 21 via a relay 20 after it has passed through a drive amplifier 19. As a result, the track servo system is closed and thereby a servo loop is constituted.
Although the servo loop acts in a normal state, the drive current output from the drive amplifier 19 is excessively increased (the servo has been removed) if defects such as dust or a damage present on the optical card 1 or external vibrations or an impact exceeds an allowable level. As a result, the excessively large electric current continues to pass through the coil of the track actuator 21, causing the coil or its support member to be burnt due to the heat generated in the coil.
Accordingly, there has been a conventional structure as disclosed in Japanese Patent Publication No. 63-33207 in which the DC component of the drive current is detected by a DC voltage detector 27 so as to be subjected to a comparison with predetermined comparison levels VRH and VRL in comparators 24a and 24b. Furthermore, if the DC component deviates from the above-described level, the limit signal is made to be active with respect to a CPU 25, the CPU then immediately turning off the relay 20 so as to cut the supply of the electric current to the track actuator 21, whereby the actuator is protected.
According to the above-described conventional structure, the excessive rotation of the tracking mirror is prevented by detecting the DC component of the drive current.
If the servo has been removed because the dust or the damage present on the optical card 1 or the external vibrations or the impact exceeds the allowable level, the drive current continues to increase in one direction, causing two cases to occur: a case in which the DC component of the drive current increases and a case in which the direction in which the drive current passes alternately changes because the servo loop oscillates.
In the former case in which the DC component increases, the abnormality can be detected so as to protect the actuator according to the above-described conventional structure disclosed in Japanese Patent Publication No. 63-33207. However, since the DC component is substantially zero in the latter case in which the servo loops oscillates, the abnormality cannot be detected by the above-described conventional structure. Therefore, there arises a fear that the coil can be burnt.
Furthermore, if the generation of the oscillation cannot be detected during the recording of information, the recording is continued in an undesirable state in which the light beam spot deviates from a desired position. Therefore, in a case where the information recorded at the deviated position is reproduced, the obtainable signal level is unsatisfactorily low, causing a reproduction signal displaying a low S/N to be generated. As a result, problems arise in that read error will easily take place and reading cannot be performed.